1. Field of the Invention
The present invention relates to chassis systems for vehicles, in particular suspension devices, and more particularly the guidance of the wheels. Suspension devices have two main functions which must be fulfilled simultaneously at any moment during operation. One of these functions is that of suspending the vehicle, that is to say, permitting substantially vertical oscillations of each wheel in accordance with the load applied to the wheel. The other function of these devices is that of guiding the wheel, that is to say, controlling the angular position of the wheel plane.
2. The Related Art
The term xe2x80x9cwheel planexe2x80x9d refers to the plane, associated with the wheel, which is perpendicular to the axis of the wheel and which passes through the center of the contact area with the ground. The angular position of the wheel plane with respect to the body of the vehicle is defined by two angles, the camber angle and the steering angle. The camber angle of a wheel is the angle separating, in a transverse plane perpendicular to the ground, the wheel plane from the mid-plane of the vehicle. This angle is positive when the upper part of the wheel deviates from the mid-plane towards the outside of the vehicle, this being commonly termed xe2x80x9ccamberxe2x80x9d or xe2x80x9cpositive camberxe2x80x9d. Conversely, when this angle is negative, the term used is xe2x80x9ccounter-camberxe2x80x9d or xe2x80x9cnegative camberxe2x80x9d. The steering angle of a wheel is the angle separating, in a horizontal plane parallel to the ground, the wheel plane from the mid-plane of the vehicle.
On most vehicles, the camber angle (xe2x80x9ccamberxe2x80x9d or xe2x80x9ccamber anglexe2x80x9d will be used without distinction hereinbelow) is fixed for a particular position of the suspension and the steering; that is to say, that theoretically it cannot vary independently of the suspension deflection or the steering. However, it undergoes variations induced by the deformations of the elements constituting the suspension device caused by the forces exerted on the wheel by the ground. These variations may be considerable. For example, an ordinary passenger car experiences camber variations of several degrees under the transverse forces developed on the tire on a curve, irrespective of the contribution of the rolling of the vehicle body (which generally tilts in the same direction under the effect of centrifugal force). This xe2x80x9celasticxe2x80x9d variation of the camber causes the camber to increase (the camber tends towards positive values) for the outer wheel on the curve. Conversely, the camber decreases (it tends towards negative values) for the inner wheel on the curve. For a long time, these predictable variations have been incorporated in the design and adjustment compromises of the suspension devices of ordinary vehicles in order to limit the harmful effects which they have on the functioning of the chassis system.
The camber has a great influence on the behavior of the vehicle and the performance of the chassis system. In particular, the performance of a tire is very variable depending on the configuration of its contact area with the ground, which configuration depends largely on the camber. The choice of the static camber angle is based mainly on these variations. Thus, for example, a large negative static camber is generally introduced on a racing vehicle in order to compensate for the variations due to the deformations of the tire under transverse force, as well as the suspension elements, even though they are much more rigid than on passenger cars, and due to the rolling of the body. This configuration is both useful and acceptable in racing, since the criteria of grip on cornering are a major concern here. In contrast, on a passenger car, since the wear of the tires and the straight-line stability have more weight in the compromise being sought, a very slightly negative initial static camber is chosen. It is necessary to accept reduced slip thrusts when the deformations of the tire and the elements of the ground contact system under the lateral forces have their effects on the positioning of the wheel plane added to the effects of the rolling of the vehicle.
In order to optimize the camber, in particular during transverse accelerations, suspension devices whose camber varies in accordance with the vertical deflection of the wheel have been designed. In this way, the rolling experienced by the body of the vehicle can induce a useful variation of the camber which partly or totally compensates for the inclination of the body of the vehicle and the deformations described above. This is the case of the so-called xe2x80x9cmulti-linkxe2x80x9d systems. These devices require a specific design and vehicle architecture, which cannot be implemented on most current vehicles for reasons of space requirement and cost. These systems react only to the consequence (deflection, rolling) of a transverse acceleration and not to the forces which cause it, thereby, on the one hand, delaying the effect of the correction. Moreover, to permit a sufficient variation of the camber, the kinematics of these systems require displacements of the position of the contact area with respect to the vehicle, called xe2x80x9ctrack changesxe2x80x9d, and these variations can also create difficulty. The range of camber corrections made possible by such systems is therefore relatively limited when the compromise necessary for the correct functioning of the other load cases, such as travelling on a bumpy road, unilateral or in contrast simultaneous bouncing, is to be observed.
From the point of view of kinematics, in terms of degrees of freedom, suspension devices generally have only one degree of freedom (of the wheel or wheel carrier with respect to the vehicle). This degree of freedom permits vertical suspension movements which, as explained above, can be combined with limited camber variations.
Systems are known, however, in which the control of the camber is active; that is to say, the geometry modifications are controlled by movements of actuating cylinders, as described, for example, in the United States patent documents U.S. Pat. Nos. 4,515,390, 4,700,972 and German patent document DE 19717418. In these systems, at least a certain degree of additional freedom controlled by actuators has been permitted. These systems are very specific, since they cannot be used in the most ordinary vehicles, in particular because of their space requirement and the considerable power necessary for the actuators.
An object of the invention is a device of simple construction, which allows control of the camber without added energy, or with limited added energy, substantially independently from the vertical oscillations of the suspension and, more generally, of the movements of the body of the vehicle, and which makes it possible to minimize the track variations.
This object is achieved by a support device which is designed to connect a wheel to suspension elements of a vehicle, wherein the wheel, of a radius xe2x80x9cRxe2x80x9d, is designed to be supported on the ground, and the support device comprises camber means which provide the wheel with a degree of freedom of camber relative to the suspension elements. The support device is configured such that, about a mean position, the wheel allows a first instantaneous center of rotation, which is situated in an interval ranging from 0.5 R above the ground to R beneath the ground. In fact, this support device replaces the rigid wheel carrier according to the state of the art. The term xe2x80x9csuspension elementsxe2x80x9d means the elements which assure that the load is carried and impart the generally vertical suspension movement to the wheel, such as the arms, springs, shock absorbers or anti-roll connections.
Preferably, the first instantaneous center of rotation is situated beneath the plane of the ground.
Also preferably, the first instantaneous center of rotation is situated transversely beneath the contact area.
According to one embodiment, the support device is configured such that it is close to equilibrium in the mean position, in the absence of transverse force exerted by the ground on the wheel in the contact area. This equilibrium can be unstable equilibrium.
Preferably, the first instantaneous center of rotation is situated substantially in the plane of the wheel.
Preferably, the support device according to the invention comprises a wheel carrier, and is designed to be connected to an intermediate support, the intermediate support constituting one of the suspension elements.
According to one embodiment, the wheel carrier is connected to the intermediate support by connection rods which are configured so as to allow the camber movement of the wheel carrier by instantaneous movement of rotation of the wheel carrier relative to the intermediate support.
According to another embodiment, the degree of camber freedom is provided by resilient deformations of deformable elements which connect the wheel carrier to the suspension elements.
The suspension elements may comprise a Mac Pherson strut.
The support device according to the invention can additionally comprise means for control which can affect the camber of the wheel. These means for control can comprise a resiliently deformable element which opposes the camber movement, this resiliently deformable element preferably consisting of elastomer articulations.
The invention also relates to a suspension device for a vehicle, comprising the above-described support device.
Such suspension device is designed to connect a wheel carrier to a body of a vehicle, which wheel carrier is designed to support a wheel, of a radius xe2x80x9cRxe2x80x9d, designed to be supported on the ground through a contact area, and comprises means which provide the wheel carrier, relative to the body, with a degree of freedom of camber and a degree of freedom of suspension movement which are independent from one another. The suspension devise is configured such that the camber movement of the wheel carrier, about a mean position, relative to the body, allows a second instantaneous center of rotation, which is situated in an interval ranging from 0.5 R above the ground to R beneath the ground. The suspension device according to the invention comprises two degrees of freedom, which permit independent movements of suspension and camber. The camber movement of the wheel (or of the wheel carrier) takes place about a second instantaneous center of rotation, which is situated at a limited distance from the contact area, in order to limit the track variations during the process of camber or counter-camber and to limit the energy necessary in the case of active control of the camber.
According to a preferred embodiment, the instantaneous center of rotation is situated in an interval ranging from 0.2 R above the ground to 0.4 R beneath the ground, and more preferably, from 0.1 R above the ground to 0.3 R beneath the ground.
In order to assure stable functioning, the device is preferably configured such that it is close to equilibrium in the mean position in the absence of transverse force exerted by the ground on the wheel in the contact area. More preferably, it is configured such that, in the absence of variations of camber, the transverse force exerted by the ground on the wheel in the contact area, generated during the suspension spring movement, does not exceed a limit corresponding to 0.3 P, where P is the weight of the vehicle.
A preferred embodiment of the invention comprises an intermediate support, which is connected firstly to the body and secondly to the wheel carrier, and wherein the connection of the intermediate support to the wheel carrier permits the degree of freedom of camber and the connection of the intermediate support to the body permits the degree of freedom of suspension spring movement.
In order to permit passive functioning, the second instantaneous center of rotation of the movement of the wheel carrier relative to the body of the vehicle can preferably be situated beneath the plane of the ground, so that transverse forces exerted by the ground on the wheel, in the contact area, induce inclination of the wheel carrier relative to the body in the direction of a decrease of camber, when the transverse forces are directed towards the interior of the vehicle, and in the direction of an increase of camber, when the said transverse forces are directed towards the exterior of the vehicle. In this case of passive functioning associated with the transverse forces, the device can comprise a means for measurement of the camber movement of the wheel carrier, in order to deduce the transverse forces.
In certain conditions, it may be necessary or advantageous to provide in addition means for control which can affect the camber of the wheel. These means can comprise a resiliently deformable element which opposes the camber movement, the deformable element consisting, for example, of elastomer articulations.
Preferably, the degree of freedom of camber can be controlled by an active means, according to running parameters of the vehicle.
Finally, the invention relates to a vehicle which is equipped with such a suspension device.